1. Field of the Invention
The present invention relates generally to a part or jig for a gas carburizing furnace.
2. Description of the Prior Art
Carburizing treatment has been used as a surface hardening treatment for a metal, particularly as a treatment for causing carbon to penetrate and diffuse in a surface of a low carbon steel. When the quenching and tempering of a low carbon steel are carried out after carburizing, only the surface layer thereof is hardened to produce a carburized part consisting of an abrasion-resistant surface layer and a core portion having a considerable toughness. Such carburizing, quenching and tempering have been not only applied to low carbon steels, but they have been also applied to various parts and materials in various fields as a heat treatment for improving abrasion resistance.
The carburizing treatments are classified into gas, liquid and solid carburizing methods on the basis of the methods for feeding CO. Typically, the gas carburizing method has been carried out. The gas carburizing method is usually carried out in a gas carburizing furnace. The carburizing methods are also classified into a batch-type carburizing method for carrying out the carburizing every charging of a raw material, and a continuous-type carburizing method for carrying out the carburizing over a period of 24 hours. Both methods have the same principle. In addition, the gas carburizing furnaces include three types of furnaces in which only carburizing, both of carburizing and quenching, and all of carburizing, quenching and tempering are carried out, respectively. Throughout the specification, the term "gas carburizing furnace" means any one of the three types of furnaces.
FIG. 10 schematically shows an example of such gas carburizing furnaces. A material to be treated is housed in a basket or the like and fed into a portion 4, which is arranged in a furnace 3 comprising a refractory 1 and a steel shell 2, by means of a mesh belt, a hearth roll or the like. As described later, a carburizing gas is fed into the furnace. The fed carburizing gas is heated by an electric heater element 6 housed in a heater tube 5, and stirred by a fan 7, so that the gas carburizing into the material to be treated is carried out.
Thus, in a gas carburizing furnace, a gas carburizing and quenching furnace or a gas carburizing, quenching and tempering furnace, there are used various metal parts, such as a radiant tube, an electric heater tube, a furnace fan, a mesh belt, a hearth roll, a pusher, a retort, a muffle, a chain guide rail, a skid rail, a roller, a thermocouple protective tube, a heater, a gas sampling tube and a stud bolt nut. In addition, in order to introduce a treated material to be gas-carburized and quenched into the furnace, there are used various metal jigs, such as a tray, a basket, a holder, a grid, a wire gauze, a vertical rod, a crossbar and a bracing.
The aforementioned parts and jigs for the gas carburizing furnace, the gas carburizing and quenching furnace or the gas carburizing, quenching and tempering furnace, are exposed to an atmosphere of carburizing gas (RX gas) mainly containing CO, H.sub.2 and N.sub.2 gases at a high temperature of 800 to 1,000.degree. C. for a long time, and used in a very severe environment wherein heating and quenching are frequently carried out. Therefore, the aforementioned parts and jigs are generally made of austenitic stainless steels or heat-resistant cast steels, which have a superior strength at elevated temperatures, a superior carburization resistance and a superior oxidation resistance at elevated temperatures. However, even if such materials are used, cracking, deformation and decreased thickness are easily caused as described below. As a result, there are problems in that the maintenance costs are increased due to their short life and the production efficiency is lowered due to facility troubles. In order to eliminate such problems, no effective measures have been taken.
(1) Cracking
As the progress of carburizing, a hard, brittle carburized layer is deeply formed in a part or jig. As a result, the thermal expansion and contraction stress caused by heating and quenching easily produce cracks, and then, cause ruptures, so that the part or jig can not be used. Many parts or jigs for a furnace have welded constructions. In the case of a welded structure, a rupture is easily caused particularly from a heat-affected zone (HAZ) of a weld, and the rupture further promotes the deformation, so that various troubles are caused.
(2) Deformation
(a) When the carburizing progresses, chromium carbide or cementite is formed in a deep carburized layer in whole, to cause a considerable deformation due to volume expansion.
(b) Since the parts and jigs for the furnace have an internal stress due to welding, plastic working or the like when it is manufactured, the parts and jigs are easily deformed due to the heat produced when they are used.
(c) The jigs are deformed in the furnace by the loads of the products loaded thereon and/or the loads of the stacked jigs.
(d) The parts and jigs for the furnace are deformed by the thermal expansion and contraction stress caused by heating and quenching. When the deformation progresses, (i) the setting of products on the jigs, the assembly of the jigs and the conveyance are not smoothly carried out, so that the working efficiency is lowered. When the deformation further progresses so that the jigs can not be used, (ii) if it is tried to rectify the deformation, there is a problem in that the jigs are ruptured since the carburizing causes the jigs to be brittle.
(3) Decreased Thickness
(a) In the case of an electric heater tube or a radiant tube, it has a short life since the thickness on the side of the inside face is decreased by oxidation and the embrittlement occurs on the outside face due to carburization. In particular, since the inside face is exposed to an oxidizing atmosphere at a temperature of higher than 1,000.degree. C., the thickness is quickly decreased by oxidation to shorten the life, and the oxidized scale is removed to accumulate in the tube. Therefore, in the case of an electric heater tube, the heater and the tube are conducted to each other via the scale to cause a burning trouble, and in the case of a radiant tube, the flow of a combustion gas is deteriorated to cause a localized heating due to abnormal combustion to cause a tube burst trouble. In addition, in order to prevent such troubles, the facility must be frequently stopped to remove the scale in the tube, so that the productivity is lowered and the maintenance costs are increased.
(b) After carburizing, quenching and tempering, the shot blasting is sometimes carried out while the product is set on the jig. In such a process, the jig is easily worn and deformed so as to decrease the life.
In view of the aforementioned problems, the inventors have found that there is a possibility of greatly extending the life by: (1) suppressing the carburization and oxidation; (2) decreasing or removing heat-affected zones of weld; (3) removing the internal stress of the part or jig for the furnace before it is used; (4) improving the abrasion resistance; and (5) improving the strength at elevated temperatures. As heat resisting alloys having a superior carburization resistance and a superior oxidation resistance, many materials are disclosed in, e.g., Japanese Patent Laid-Open Publications Nos. 7-166290 and 2-259037. However, these materials contain only a small amount (not more than 3 wt %) of Al, which is an element that is the most effective for the improvement of carburization resistance and oxidation resistance. If the material contains a great amount of Al, although the carburization resistance and oxidation resistance can be improved, the material becomes brittle. Therefore, there are problems in that it is difficult to carry out the plastic working and the material is easily cracked when it is used. In addition, the molten metal is difficult to flow when the material is cast. Moreover, there is also a problem in that it is impossible to weld the matter. Therefore, it is not possible to add a great amount of Al as a component. On the other hand, there is a method for improving the carburization resistance and oxidation resistance by performing the surface treatment of an austenitic stainless steel or a heat-resistant cast steel to form a protective film on only the surface. However, protective films formed by general surface treatments, e.g., plating, thermal spraying, vapor deposition, are immediately peeled off in a severe thermal shock environment such as a gas carburizing furnace, so that the extension of the life can not be expected.